Crash when starting game

[Swholli]

Whenever I try to host a game, even on single player, it dies....  (see attached screen shot) anyone know why? I've recently reformatted and reinstalled the game... is it an addon perhaps? A decal? I'm really stuck here, cause I like to host servers but can't any more. Please help, much obliged.

~Scott

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[Swholli]

Sorry Badspot for changing my ambiguous title, but that still doesn't answer my question... perhaps you would know the error? It's never done that before... until recently...